Another variation on what John said, from some one over at WTR: "Never take a finished piece into the house from the shop on a sunny day. Sunlight causes scratches." Been there, done that, more than once.... So, how to get them all out??? You have to be able to see them first. This involves good glasses and proper lighting. As far as glasses, the cheap reading glasses are about a 5 on the 10 point scale, unless you just barely need them. Prescription glasses made a huge difference for me. As for lighting, I prefer the light spectrum of the 'therapy' lamps, which are almost all florescent types rather than the LED lights which all seem to be in the white spectrum. Our eyes evolved to see in natural sun light. Ott lamps are one that all the needle point and hand quilters use. Most of those lamps are not bright enough or don't cast a broad enough beam for bowls, but for smaller pieces would work fine. I have been using Blue Max floor lamps, also used by hand sewers, which have a 65 watt (I think), 3 long loops type of bulb in them. I can see just about every thing with them. These are also good for grey rainy days in western Oregon when you need a little sun light fix... Other than that, it is being able to tell when you have all of the scratches from the previous grit sanded out. You will also spend more time sanding with the coarser grits than you do with the finer grits. I avoid 80 grit when ever possible, which now is most of the time. If I do need it, it is because I have turned some thing that had patina to it as it dried, like sap wood on cherry, and I have to sand it down to white. I follow that with 100 grit. Some times the 80 grit marks are harder to remove than the tool marks... After 120 grit, I start to try to make a contrasting scratch pattern. So, on the outside of a bowl, and with the power sander/drill, I will sand using the disc on the 10 o'clock part of the pad, and then next grit switch to the 2 o'clock part of the pad so the scratch marks will criss cross each other. I use a firm/hard pad up to about 180 or 220 grit, medium pad up to 400, and if I go over that, then I switch to a soft pad. I never blow off a piece when sanding. In part, I don't want to spread dust around. There are some who proclaim that they do it to make sure there are no remaining grit pieces stuck in the wood, which cause the 'mystery' scratches, which seems to explain why they turn up... Well, instead of blowing out the bowl, I wipe it off with my hand, usually in the 180 grit stage. Out of all the thousands of bowls I have sanded and hand wiped, I have never felt a single grit of abrasive stuck in the wood. I am guessing that the mystery scratches come from not getting all the grit scratches out from the previous grit before stepping up to the next grit. Hand rubbing the bowl also seems to push the finer dust into any remaining scratches, which actually high lights them rather than hiding them.
One other cause of mystery scratches can come from the discs and the pads. If I use a 3 inch disc on a 3 inch pad, even if it is perfectly centered, the plastic backing for the hooks can leave 80 grit type scratches. Same if I am using still paper or cloth discs. The edges are hard and don't flex, and especially on the inside of a bowl, they can leave deep marks of the "hey, that wasn't there before I started with the last grit, what happened???"
Of course, some times we just aren't paying close enough attention...
robo hippy